The present invention relates to a system for taking backup in a distributed real time data base in operation, which is located on primary memory.
This application claims priority from Swedish patent application no. 9203691-2, filed Dec. 8, 1992, which is expressly incorporated here by reference.
A data base normally has a recover functionality responsive for data stored in the data base being consistent, i.e., being in a state to be expected with respect to different up-dates.
For enabling recovery from errors write-damaging the data base, a backup version needs to be stored. A backup includes all information necessary for being able to recover the data base to the same consistent state that prevailed at the time when the backup version was created.
A typical strategy for recovering a data base can be summarized as follows.
If the data base has become inconsistent due to a catastrophic error having occurred, the recovery function orders restart with reloading of the data base to the latest backup version. In order to enable return to a more current state, the data base is then updated with information that has been stored in a log, i.e., a history of earlier operations stored in a memory. The log is used by the recovery function for restoring the latest allowed transactions, i.e., operations on objects or records in the data base.
The term "object" is here intended to have the conventional meaning within the data processing art. In short, an object is an information packet with an associated description of how to manage its information. The objects are divided into classes after their area of use, and are identified by means of object identities. A more detailed description of the concept of object can be found, e.g., in "OBJECT-ORIENTED ANALYSIS" by Peter Coad and Edward Yourdon, Yourdon Press Computing Series.
The information and mechanisms required for recovery of the data base are determined by the method chosen for updating the data base. In case of updating an object a copy of the original object is created, a lock is set for preventing other users from updating the data base, and at commit, i.e., the transaction operation used by a program or an operator for indicating that a current transaction has been ended and that its effects should remain, the updated copy is switched in and set locks are released.
Copied objects or objects which shall not be submitted to backup are erased at restart of the database with reloading.
Within the current technical area it is highly desirable that the data base system not stop during backing up.
In U.S. Pat. No. 4,077,059 a system is described having a hierarchic memory with two memory units on each level. One of the units contains all data on this level. The other unit contains only changes which have been made to these data. By duplicating and recovering only changed data, the amount of data which must be transported in case of errors, is decreased.
The patent does not describe a data base system in a strict sense. There is no backing up functionality activated by a central handler which has information about the system and synchronizes the functionality.
U.S. Pat. No. 4,714,995 describes an integrated processor system with associated data bases. Integration is done of processor systems having a need of sharing certain common data elements, where each processor system has a data base. The object is to perform controlled copying. Relations and transactions are allowed to extend over several data bases.